A compact LED lighting unit can for example be considered to be a lighting unit with a package height less than 3 mm, and an output aperture with an area of less than 100 mm2. A compact lighting unit of this type can be integrated into a portable device such as a mobile phone.
Compact LED lighting units such as flash LED packages typically consist of high power LEDs to generate white light, usually at a colour temperature of about 5500-6500 K, combined with Fresnel optical lenses either through integration with the LED package directly or with a separate housing for the lens and the LED package.
These packages typically use a high power blue LED covered with a phosphor layer which converts a part of the radiation into the green-red spectral range to result in a white color point. The blue LED has typically a 1 mm2 size and is mounted on a ceramic support substrate. The total outer dimensions of the LED substrate are for example typically 1.6×2.0 mm. Some packages appear very yellow due to the phosphor layer; other packages are made to appear much whiter by molding white scattering material on top and around the LED phosphor. Customers tend to dislike the yellow appearance when magnified by a Fresnel lens and hence an off-state-white (OSW) appearing package is sometimes preferred even though the efficacy is reduced by the extra white scattering layers.
Flash LEDs of this type are for example applied as flash units in mobile phone applications, such as disclosed in US20100178046. To concentrate the light on a 4:3 or 16:9 scene that is captured by the camera, the light of the flash LED, which initially has Lambertian angular distribution of intensity, is collimated by the Fresnel lens. This can be achieved by clamping the LED package and a separate Fresnel lens into a package assembly, for example with a typical height of about 3 mm. As mentioned above, the Fresnel lens can instead be combined with the LED package onto a thin PCB as one integrated, more compact unit.
FIG. 1 shows a compact LED lighting unit 1 with a Fresnel lens optical structure on the top surface. The lens is molded around the LED package on a thin PCB and this can for example achieve a reduced build height of 2.2 mm.
One example of Fresnel lens comprises optical elements in the form of facets that refract light in the center of the lens, and reflect light upwards at the periphery of the lens in order to collimate the light emitted by the LED. All-refracting lenses are also possible.
The Fresnel lens needs to be located at a certain minimum distance from the LED in order to operate properly as a lens and a careful alignment of the position of the LED to the optical center of the Fresnel lens is needed.
The build height of the flash LED with Fresnel lens is very important to mobile phone makers because it is a discriminating trend to make mobile phones thinner and thinner and use less and less space for the components inside the phone, especially in the depth direction. Breakthroughs in build height of the components in the phone are therefore very important.
In addition to physical dimension limitations, there are specific requirements to the beam profile that is emitted by a flash LED package. These requirements relate to the illumination of a scene captured by the camera and can for example be measured by projecting the light on a screen. For example, the light can be captured on a screen with the desired aspect ratio (e.g. 4:3) at 1 m from the flash unit, and regions of the screen can be analysed, for example dividing the screen into 21×31 analysis regions.
The main requirements for a Field of View (FOV) in the range 70-75 degrees are:
Light uniformity on the screen of more than 10%, preferably more than 15%, more preferably more than 20%, and even more preferably 30% or more (i.e. the minimum illuminance in the screen corners is at least the specified percentage of the maximum illuminance in the screen center;
Illuminance at screen center divided by total luminous flux >0.6, more preferably >0.7. Typical lumen output levels are 260 lm or more at 1 A pulse operation; resulting in typically more than 170 lux in the center of the screen;
Full Width Half Maximum (FWHM) of the beam profile about 65-75 degrees;
Correlated colour temperature (CCT) typically 5500-6500 K.
The light uniformity for a flash mentioned above is defined in general as the average luminous flux (“lux”) at the corners of the screen divided by the lux in the center of the screen. FIG. 2 shows how the flash specifications are defined, and shows the flash unit 1 projecting onto a screen 2.
The uniformity is defined as the central lux of the screen (central bin: bC) divided by the average corner lux (corner bins: b1, b2, b3, b4).Uniformity=bC/(0.25*(b1+b2+b3+b4))
The central lux (central bin, bC) divided by total emitted flux of the flash module (including the part that is not incident on the screen) is typically 0.6-0.7 lux/lm. A typical setup is a field of view of 73 degrees along the diagonal, with the screen 1 m from the flash module and a screen ratio of 4:3.
The flash unit using a known Fresnel lens design has several limitations. There is a fundamental distance required between the Fresnel lens and the LED source as indicated above. To make the package thinner, the lateral dimensions of the lens and LED package need to be scaled down. This means a smaller LED, which gives limitations to the amount of light that can be generated. For instance, for a typical LED die of 1×1 mm2 the build height with a Fresnel lens is limited to about 2 mm.
LEDs emit in general with a Lambertian angular intensity distribution. Without any additional optics between the LED and the Fresnel lens, which would make the flash module bigger, the acceptance angle of the Fresnel lens is dictated by the distance from the lens to the light emitting surface and the diameter or lateral size of the lens. This is basically the numerical aperture of the Fresnel lens. Light that is emitted outside of this acceptance angle (aperture) is not useful and is in general lost. Placing a Fresnel lens closer and closer to the LED to capture more and more light is fundamentally not possible as this requires materials with higher and higher refractive indices which are not economically viable or not available.
Because of the small package size that is required there is very little possibility to extract the light from the LED efficiently. Therefore the package usually consists only of a phosphor layer that directly covers a blue LED, limiting efficiency. The Fresnel lens also enlarges the yellowish appearance from the phosphor on the LED, which is disliked by customers. To partly compensate this yellowish appearance, a white scattering layer can be employed on the LED emitter, with the drawback of reduced efficiency.